Heyboer vs Mercury Magnetics. What's the difference? And how should I choose?

Heyboer and Mercury Magnetics are transformer manufacturers. Each offers power, output and chokes for Marshalls and many other amplifiers. Actually, several versions of the parts found in classic amps over the years. Take the JTM 45 output transformer, for example. There were many different types found stock from at least two different suppliers. The Radio Spares output came in nearly a dozen different configurations alone.

Both companies have compiled specs from these parts first hand by tearing down blown examples for rewinding. Each part, material, wire, gauge and interleave is documented and recreated. MM and their absolute genius leader, Sergio have probably documented and recreated more of these classic transformers than most other companies combined. To call them transformer experts would be an understatement.

So, if everyone recreates these parts from complete teardowns, why are they all different?

Several reasons. For one, these parts varied enough that unless two companies recreated the exact same part, you would find (and hear) differences.
Secondly, documentation and part numbering often left much to be desired. It's hard to know that you are comparing recreations of the same part.
Thirdly, manufacturing process. Modern machines are limited in most cases to even numbers of alternating laminations and only four interleaved windings (without additional set up). Thus, being completely accurate is expensive and labor intensive. It's easy to see why a transformer with a hand assembled core and several interleaved winds costs several times what a typical machine made part costs.
Lastly, materials. Since these parts were originally made, steel and copper have changed. Modern materials are more precise and often higher quality. Today’s M6 steel is more efficient than it's 35-year-old equivalent. Which changes the core characteristics. Great for cool running power transformers, but not so good for output transformer cores where you want them to saturate in a musical way.
Other modern materials such as plastic and Teflon have replaced paper and fish paper. Which affects tone.

Transformer manufactures have to consider these (and many other) factors when "recreating" classics. One approach is to use modern materials and compensate for their tonal affects. MM parts use modern materials, but Sergio compensates for core saturation and other technical details. Things beyond the scope of this. Heyboer, by comparison uses closest modern equivalents of classic materials. Heyboer will also build to any specs you supply. As in the case of my Dagnall C1998 clone output transformer.

For my Dagnall clone, I used two examples of this output. One from Nov/Dec '67, a very early example. And a second from mid '69.
Not being a transformer designer or engineer, recreating this part took several trial and error prototypes and critical listening sessions. In the end, exact wind and interleaving combined with custom stamped and hand annealed steel, paper bobbin, self leads and many other details were required to make the recreation behave and sound accurate to the originals.

Getting back on topic re: Heyboer vs MM.
Which one sounds more like the originals? Both are very good, and miles beyond typical stock transformers. I would say they are different takes on the same theme, when comparing recreations of the same vintage part.
The MM parts tend to have a more open, extended high end to my ears. Very complex tone, notes and harmonics jump out from under your fingers. They sound very alive. I also find them to feel faster compared the Heyboer parts. Which tend to sound and feel blusier, for lack of a better term. Slightly slower and more like a transformer that has aged 35 years. Both parts share a musical, three dimensional characteristic.

Is it worth the extra cost for the MM? Absolutely. You're paying for hand assembly in many cases, tonal expertise and the piece of mind of knowing that these guys have seen every variation of classic iron.
Not to discount Heyboer, though. Their off the shelf parts are still a major upgrade over stock components.

How should I decide which is right for me? If there was an easy answer, this would have been a much shorter article. But there are some things I can mention to help point you in the right direction:

Do you want a JTM45 output? Or a Radio Spares deluxe replica, from early '65 with 6.6k primary, tag board and 16 only tap? If you need a very specific part, MM may be the best option. They have dozens of variations of most parts and can suggest the most accurate part for a specific amp or model.
If you want to build a mid 60's JTM 45 clone, either brand can get you there. With extremely good results.

Are you a builder/hobbyist who knows tranny specs and wants to experiment? If so, call Heyboer and give them your specs. They can build what you want at a fair price and you can tweak from there.

Do you just want the best EVH output transformer money can buy period? If so, get the O100JM-SL from MM and spend several hours a day working on your technique.

Do you have a classic amp with a blown part that you want to replace and not change the tone? The bad news is that nothing really sounds like a 35-year-old tranny. Especially one that has been on it's last leg for a while. Fortunately, either of these parts will get your amp sounding like it would have sounded earlier in it's life.
Once again, if you need a very specific part MM is more likely to have it documented and on the shelf.

Are you building a kit? In this case select whichever part appeals to you more. You can't go wrong with either brand. And you'll need to refine and upgrade other parts in the kit considerably before the transformers are the weakest link. At which point you'll have a truly world class amp.

I hope this helps and answers more questions than it raises. Let me close by saying that I am not a transformer designer or engineer. But I have good ears and I try to represent these companies and their products as accurately and sincerely as possible.
My opinions come from using these products personally and selling 100's of them over the years. Also from comparing them to originals, which is what I consider the benchmark for any recreation.
I encourage everyone to educate yourself and make informed choices. Your best source for info is to contact these companies directly. Both have excellent customer service and are happy to answer your questions.
Don't forget that there are many other transformer options available too. OEI, Marstran, Mojo, Weber ect.

so i wonder - i kind of look at MM OT as one that has lots of highs and harmonics - perhaps almost in a compensating way ( added ) compared to a heyboer which maybe alittle more "true" and ya really get what ya get out of it based on the spec.

There are basically 2 factors, which are causing sag in/across the PT.

1) The secondary's - but also the primary's - winding resistance. After Ohm's law the voltage drop across the winding is the product of copper resistance * pulled current. So the more current you're pulling out of the PT, the more voltage is going lost internal across the PT's windings.

2) The iron - means, the core's dimension, the sheet metal's quality AND the accuracy, how proper the sheet metal has been fitted into the bobbin. As soon as the core is driven into saturation, the secondary's voltage is dropping down overproportional, because the PT has reached its limit - and when you're pulling still more current, the power doesn't rise anymore, because that's the product of current and voltage.

Shure, there are some more factors - but it would be too complex and confusing, to explain it all.